Golf balls utilized in tournament or competitive play today are regulated for consistency purposes by the United States Golf Association (U.S.G.A.). In this regard, there are five (5) U.S.G.A. specifications which golf balls must meet under controlled conditions. These are size, weight, velocity, driver distance and symmetry.
Under the U.S.G.A. specifications, a golf ball can not weigh more than 1.62 ounces (with no lower limit) and must measure at least 1.68 inches (with no upper limit) in diameter. However, as a result of the openness of the upper or lower parameters in size and weight, a variety of golf balls can be made. For example, golf balls are manufactured today which by the Applicant are slightly larger (i.e., approximately 1.72 inches in diameter) while meeting the weight, velocity, distance and symmetry specifications set by the U.S.G.A.
Additionally, according to the U.S.G.A., the initial velocity of the ball must not exceed 250 ft/sec. with a 2% maximum tolerance (i.e., 255 ft/sec.) when struck at a set club head speed on a U.S.G.A. machine. Furthermore, the overall distance of the ball must not exceed 280 yards with a 6% tolerance (296.8 yards) when hit with a U.S.G.A specified driver at 160 ft/sec. (clubhead speed) at a 10 degree launch angle as tested by the U.S.G.A. Lastly, the ball must pass the U.S.G.A. administered symmetry test, i.e., fly consistency (in distance, trajectory and time of flight) regardless of how the ball is placed on the tee.
While the U.S.G.A. regulates five (5) specifications for the purposes of maintaining golf ball consistency, alternative characteristics (i.e., spin, feel, durability, distance, sound, visibility, etc.) of the ball are constantly being improved upon by golf ball manufacturers. This is accomplished by altering the type of materials utilized and/or improving construction of the balls. For example, the proper choice of cover and core materials are important in achieving certain distance, durability and playability properties. Other important factors controlling golf ball performance include, but are not limited to, cover thickness and hardness, core stiffness (typically measured as compression), ball size and surface configuration.
As a result, a wide variety of golf balls have been designed and are available to suit an individual player's game. Moreover, improved golf balls are continually being produced by golf ball manufacturers with technologized advancements in materials and manufacturing processes.
Two of the principal properties involved in a golf ball's performance are resilience and compression. Resilience is generally defined as the ability of a strained body, by virtue of high yield strength and low elastic modulus, to recover its size and form following deformation. Simply stated, resilience is a measure of energy retained to the energy lost when the ball is impacted with the club.
In the field of golf ball production, resilience is determined by the coefficient of restitution (C.O.R.), the constant "e" which is the ratio of the relative velocity of an elastic sphere after direct impact to that before impact. As a result, the coefficient of restitution ("e") can vary from 0 to 1, with 1 being equivalent to a perfectly or completely elastic collision and 0 being equivalent to a perfectly or completely inelastic collision.
Resilience (C.O.R.), along with additional factors such as club head speed, club head mass, angle of trajectory, ball size, density, composition and surface configuration (i.e., dimple pattern and area of coverage) as well as environmental conditions (i.e., temperature, moisture, atmospheric pressure, wind, etc.) generally determine the distance a golf ball will travel when hit. Along this line, the distance a golf ball will travel under controlled environmental conditions is a function of the speed and mass of the club and the size, density, composition and resilience (C.O.R.) of the ball and other factors. The initial velocity of the club, the mass of the club and the angle of the ball's departure are essentially provided by the golfer upon striking. Since club head, club head mass, the angle of trajectory and environmental conditions are not determinants controllable by golf ball producers and the ball size and weight are set by the U.S.G.A., these are not factors of concern among golf ball manufacturers. The factors or determinants of interest with respect to improved distance are generally the coefficient of restitution (C.O.R.), spin and the surface configuration (dimple pattern, ratio of land area to dimple area, etc.) of the ball.
The coefficient of restitution (C.O.R.) in solid core balls is a function of the composition of the molded core and of the cover. The molded core and/or cover may be comprised of one or more layers such as in multi-layered balls. In balls containing a wound core (i.e., balls comprising a liquid or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings. As in the solid core balls, center and cover of a wound core ball may also consist of one or more layers.
The coefficient of restitution of a golf ball can be analyzed by determining the ratio of the outgoing velocity to the incoming velocity. In the examples of this writing, the coefficient of restitution of a golf ball was measured by propelling a ball horizontally at a speed of 125+/-1 feet per second (fps) against a generally vertical, hard, flat steel plate and measuring the ball's incoming and outgoing velocity electronically. Speeds were measured with a pair of Oehler Mark 55 ballistic screens (available from Oehler Research Austin Tex.), which provide a timing pulse when an object passes through them. The screens are separated by 36" and are located 25.25" and 61.25" from the rebound wall. The ball speed was measured by timing the pulses from screen 1 to screen 2 on the way into the rebound wall (as the average speed of the ball over 36"), and then the exit speed was timed from screen 2 to screen 1 over the same distance. The rebound wall was tilted 2 degrees from a vertical plane to allow the ball to rebound slightly downward in order to miss the edge of the cannon that fired it.
As indicated above, the incoming speed should be 125+/-1 fps. Furthermore, the correlation between C.O.R. and forward or incoming speed has been studied and a correction has been made over the +/- fps range so that the C.O.R. is reported as if the ball had an incoming speed of exactly 125.0 fps.
The coefficient of restitution must be carefully controlled in all commercial golf balls if the ball is to be within the specifications regulated by the U.S.G.A. As mentioned to some degree above, the U.S.G.A. standards indicate that a "regulation" ball cannot have an initial velocity exceeding 255 feet per second in an atmosphere of 75.degree. F. when tested on a U.S.G.A. machine. Since the coefficient of restitution of a ball is related to the ball's initial velocity, it is highly desirable to produce a ball having sufficiently high coefficient of restitution (C.O.R.) to closely approach the U.S.G.A. limit on initial velocity, while having an ample amount of softness (i.e., hardness) to produce the desired degree of playability (i.e., spin, etc.).
Furthermore, the maximum distance a golf ball can travel (carry and roll) when tested on a U.S.G.A. driving machine set at a club head speed of 160 feet/second is 296.8 yards. While golf ball manufacturers design golf balls which closely approach this driver distance specification, there is no upper limit for how far an individual player can drive a ball. Thus, while golf ball manufacturers produced balls having certain resilience characteristics in order to approach the maximum distance parameter set by the U.S.G.A. under controlled conditions, the overall distance produced by a ball in actual play will vary depending on the specific abilities of the individual golfer.
The surface configuration of a ball is also an important variable in affecting a ball's travel distance. The size and shape of the ball's dimples, as well as the overall dimple pattern and ratio of land area to dimpled area are important with respect to the ball's overall carrying distance. In this regard, the dimples provide the lift and decrease the drag for sustaining the ball's initial velocity in flight as long as possible. This is done by displacing the air (i.e., displacing the air resistance produced by the ball from the front of the ball to the rear) in a uniform manner. The shape, size, depth and pattern of the dimple affect the ability to sustain a ball's initial velocity differently.
As indicated above, compression is another property involved in the overall performance of a golf ball. The compression of a ball will influence the sound or "click" produced when the ball is properly hit. Similarly, compression can affect the "feel" of the ball (i.e., hard or soft responsive feel), particularly in chipping and putting.
Moreover, while compression of itself has little bearing on the distance performance of a ball, compression can affect the playability of the ball on striking. The degree of compression of a ball against the club face and the softness of the cover strongly influences the resultant spin rate. Typically, a softer cover will produce a higher spin rate than a harder cover. Additionally, a harder core will produce a higher spin rate than a softer core. This is because at impact a hard core serves to compress the cover of the ball against the face of the club to a much greater degree than a soft core thereby resulting in more "grab" of the ball on the clubface and subsequent higher spin rates. In effect the cover is squeezed between the relatively incompressible core and clubhead. When a softer core is used, the cover is under much less compressive stress than when a harder core is used and therefore does not contact the clubface as intimately. This results in lower spin rates.
The term "compression" utilized in the golf ball trade generally defines the overall deflection that a golf ball undergoes when subjected to a compressive load. For example, PGA compression indicates the amount of change in a golf ball's shape upon striking. The development of solid core technology in two-piece balls has allowed for much more precise control of compression in comparison to thread wound three-piece balls. This is because in the manufacture of solid core balls, the amount of deflection or deformation is precisely controlled by the chemical formula used in making the cores. This differs from wound three-piece balls wherein compression is controlled in part by the winding process of the elastic thread. Thus, two-piece and multilayer solid core balls exhibit much more consistent compression readings than balls having wound cores such as the thread wound three-piece balls.
Additionally, cover hardness and thickness are important in producing the distance, playability and durability properties of a golf ball. As mentioned above, cover hardness directly affects the resilience and thus distance characteristics of a ball. All things being equal, harder covers produce higher resilience. This is because soft materials detract from resilience by absorbing some of the impact energy as the material is compressed on striking.
Furthermore, soft covered balls are preferred by the more skilled golfer because he or she can impact high spin rates that give him or her better control or workability of the ball. Spin rate is an important golf ball characteristic for both the skilled and unskilled golfer. As just mentioned, high spin rates allow for the more skilled golfer, such as PGA and LPGA professionals and low handicap players, to maximize control of the golf ball. This is particularly beneficial to the more skilled golfer when hitting an approach shot to a green. The ability to intentionally produce "back spin", thereby stopping the ball quickly on the green, and/or "side spin" to draw or fade the ball, substantially improves the golfer's control over the ball. Thus, the more skilled golfer generally prefers a golf ball exhibiting high spin rate properties.
However, a high spin golf ball is not desirous by all golfers, particularly high handicap players who cannot intentionally control the spin of the ball. Additionally, since a high spinning ball will roll substantially less than a low spinning golf ball, a high spinning ball is generally short on distance.
In this regard, less skilled golfers, have, among others, two substantial obstacles to improving their game: slicing and hooking. When a club head meets a ball, an unintentional side spin is often imparted which sends the ball off its intended course. The side spin reduces one's control over the ball as well as the distance the ball will travel. As a result, unwanted strokes are added to the game.
Consequently, while the more skilled golfer frequently desires a high spin golf ball, a more efficient ball for the less skilled player is a golf ball that exhibits low spin properties. The low spin ball reduces slicing and hooking and enhances distance. Furthermore, since a high spinning ball is generally short on distance, such a ball is not universally desired by even the more skilled golfer.
With respect to high spinning balls, up to approximately twenty years ago, most high spinning balls were comprised of balata or blends of balata with elastomeric or plastic materials. The traditional balata covers are relatively soft and flexible. Upon impact, the soft balata covers compress against the surface of the club producing high spin. Consequently, the soft and flexible balata covers provide an experienced golfer with the ability to apply a spin to control the ball in flight in order to produce a draw or a fade, or a backspin which causes the ball to "bite" or stop abruptly on contact with the green.
Moreover, the soft balata covers produce a soft "feel" to the low handicap player. Such playability properties (workability, feel, etc.) are particularly important in short iron play with low swing speeds and are exploited significantly by relatively skilled players.
However, despite all the benefits of balata, balata covered golf balls are easily cut and/or damaged if mis-hit. Golf balls produced with balata or balata-containing cover compositions therefore have a relatively short lifespan.
Additionally, soft balata covered balls are shorter in distance. While the softer materials will produce additional spin, this is frequently produced at the expense of the initial velocity of the ball. Moreover, as mentioned above, higher spinning balls tend to roll less.
As a result of these negative properties, balata and its synthetic substitutes, transpolyisoprene and trans-polybutadiene, have been essentially replaced as the cover materials of choice by new synthetic materials. Included in this group of materials are ionomer resins.
Ionomeric resins are polymers in which the molecular chains are cross-linked by ionic bonds. As a result of their toughness, durability and flight characteristics, various ionomeric resins sold by E. I. DuPont de Nemours & Company under the trademark "Surlyn.RTM." and more recently, by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademarks "Escor.RTM." and the trade name "lotek", have become the materials of choice for the construction of golf ball covers over the traditional "balata" (transpolyisoprene, natural or synthetic) rubbers. As stated, the softer balata covers, although exhibiting enhanced playability properties, lack the durability (cut and abrasion resistance, fatigue endurance, etc.) properties required for repetitive play and are limited in distance.
Ionomeric resins are generally ionic copolymers of an olefin, such as ethylene, and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic group in the copolymer resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e. durability, etc., for golf ball cover construction over balata.
Historically, some of the advantages produced by ionomer resins gained in increased durability were offset to some degree by decreases produced in playability. This was because although the ionomeric resins were very durable, they initially tended to be very hard when utilized for golf ball cover construction, and thus lacked the degree of softness required to impart the spin necessary to control the ball in flight. Since the initial ionomeric resins were harder than balata, the ionomeric resin covers did not compress as much against the face of the club upon impact, thereby producing less spin.
In addition, the initial, harder and more durable ionomeric resins lacked the "feel" characteristic associated with the softer balata related covers. The ionomer resins tended to produce a hard responsive "feel" when struck with a golf club such as a wood, iron, wedge or putter.
As a result of these difficulties and others, a great deal of research has been and is currently being conducted by golf ball manufacturers in the field of ionomer resin technology. There are currently more than fifty (50) commercial grades of ionomers available both from DuPont and Exxon, with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e., relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcement agents, etc. However, a great deal of research continues in order to develop golf ball cover compositions exhibiting not only the improved impact resistance and carrying distance properties produced by the "hard" ionomeric resins, but also the playability (i.e., "spin", "feel", etc.) characteristics previously associated with the "soft" balata covers, properties which are still desired by the more skilled golfer.
Consequently, a number of two-piece (a solid resilient center or core with a molded cover) and three-piece (a liquid or solid center, elastomeric winding about the center, and a molded cover) golf balls have been produced by the Applicant and others to address these needs. The different types of materials utilized to formulate the cores, covers, etc. of these balls dramatically alters the balls' overall characteristics.
In addition, multi-layered covers containing one or more ionomer resins have also been formulated in an attempt to produce a golf ball having the overall distance, playability and durability characteristics desired. For example, this was addressed by Spalding Sports Worldwide, Inc., the assignee of the present invention, in U.S. Pat. No. 4,431,193 where the construction of a multi-layered golf ball having two ionomer resin cover layers is disclosed.
In the examples of the '193 patent, a multi-layer golf ball is produced by initially molding a first cover layer on a solid spherical core and then adding a second layer. The first layer is comprised of a hard, high flexural modulus resinous material such as type 1605 Surlyn.RTM. (now designated Surlyn.RTM. 8940). Type 1605 Surlyn.RTM. (Surlyn.RTM. 8940) is a sodium ion based low acid (less than or equal to 15 weight percent methacrylic acid) ionomer resin having a flexural modulus of about 51,000 psi. An outer layer of a comparatively soft, low flexural modulus resinous material such as type 1855 Surlyn.RTM. (now designated Surlyn.RTM. 9020) is molded over the inner cover layer. Type 1855 Surlyn.RTM. (Surlyn.RTM. 9020) is a zinc ion based low acid (10 weight percent methacrylic acid) ionomer resin having a flexural modulus of about 14,000 psi.
The '193 patent teaches that the hard, high flexural modulus resin which comprises the first layer provides for a gain in coefficient of restitution over the coefficient of restitution of the core. The increase in the coefficient of restitution provides a ball which serves to attain or approach the maximum initial velocity limit of 255 feet per second as provided by the United States Golf Association (U.S.G.A.) rules. The relatively soft, low flexural modulus outer layer provides essentially no gain in the coefficient of restitution but provides for the advantageous "feel" and playing characteristics of a balata covered golf ball.
Unfortunately, however, while the ball of the examples of the '193 patent do exhibit enhanced playability characteristics with improved distance (i.e. enhanced C.O.R. values) over a number of other then known multi-layered balls, the balls suffer from relatively short distance (i.e. lower C.O.R. values) when compared to two-piece, single cover layer balls commercially available today. These undesirable properties make the balls produced in accordance with the limited examples of the '193 patent generally unacceptable by today's standards.
The present invention is directed to new multi-layer golf ball compositions which provide for enhanced coefficient of restitution (i.e, improved travel distance) and/or durability properties when compared to the multi-layer balls found in the examples of the prior art. The travel distance of the balls of the invention is further improved by increasing the balls' moment of inertia and thereby reducing their overall spin rate.
Moreover, the balls of the invention have enhanced outer cover layer softness and feel. The improvements in distance, feel, etc. are produced without substantial sacrifices in controllability resulting from the loss of spin produced by the balls' increased moment of inertia.
As previously noted, a low spin ball is generally preferred, particularly for the less skilled player. And, as noted, increasing the moment of inertia of the ball tends to reduce the spin rate of the ball. The present invention provides a remarkable and unique approach for readily increasing the moment of inertia of a golf ball. The approach of the present invention is economical and easily implemented in large scale commercial golf ball manufacturing processes.
These and other objects and features of the invention will be apparent from the following summary of the invention, description of the preferred embodiments, the drawings and from the claims.